Pitch production, making a high softening point material by thermal polymerization of normally liquid streams, is an ancient process. Use of pitch, for sealing baskets of reeds floating in the river, or for sealing Noah's ark, is reported in the Bible. “Make thee an ark . . . pitch it within and without with pitch.” Genesis 8:14.
With the rise of great sailing ships, made of wood, use of pitch increased. Pitch was made from sap, from charcoal and from the roots of pine trees. Pine tar was used so extensively on ships that sailors were often called “tars”, in reference to the constant contamination of their feet with tar used on decks and line. From 1720 to 1870, North Carolina was the world's leading producer of naval stores, turpentine, pitch and tar, all made from the state's abundant pine trees.
Wood tar has enjoyed a reputation as a sticky substance for over 100 years. North Carolina has a semi-official nickname of “The Tar Heel State” and the term is now one of admiration, rather than disrespect. The State Library of North Carolina reports the nickname relates to a civil war battle in which tenacious North Carolina fought on after troops from other rebel states left the field. The North Carolina troops responded to requests, from the rebel troops, asking if there was any tar left in the state, answering that Jeff Davis had purchased all the tar in North Carolina “He's going to put on you-un's heels to make you stick better in the next fight.” Creecy relates that General Lee, upon hearing of the incident, said: “God bless the Tar Heel boys,” and from that they took the name (Adapted from Grandfather Tales of North Carolina by R. B. Creecy and Histories of North Carolina Regiments, Vol. III, by Walter Clark.
The nickname is mentioned to show that for over 100 years, wood tar has been known as one of the stickiest substances around, a property which is useful for many purposes, but greatly increases the difficulty of working with the material, as will be discussed in greater detail hereafter.
While wood tar pitch was the primary pitch product for millennia, it gradually was displaced by pitch products derived from coal and, eventually, from petroleum.
All pitch production processes are similar. All start with, or produce as an intermediate product, a relatively low molecular weight, normally liquid material. Cooking pine produces pine tar, further heating of which produces wood tar pitch. Cooking coal produces coal tar, with further heating, or at least fractionation, producing coal tar pitch.
All pitch refining processes are similar whether the starting material is derived from wood, coal or petroleum. Common to all, vaporizable components are removed from non-vaporizable or non-distillable components (the pitch portion of the product). The removal of progressively more of the distillable components from the pitch fraction increases the softening point of the remaining pitch. In wood tar pitch, if too much turpentine is left in the pitch, the pitch is too soft. In coal tar pitch, if too much creosote, or other solvent, remains in the pitch, the softening point is too low. In petroleum pitch, as distillable hydrocarbons are removed, the softening point of the product pitch increases.
While distillation is basically a simple process involving heating, difficulties abound when the feed is a viscous, sticky and potentially polymerizable material like pitch. It is relatively easy to make a pitch with a lot of distillable liquids in it. Making a pitch with a softening point above 200° F., 250° F. is now practiced commercially, though it is somewhat easier to make these materials from coal tar than petroleum. This is because the coal coking process used to make coal tar produces a superheated vapor, which is cooled and fractionated to recover the coal tar fraction, while petroleum pitch requires heating to make crude pitch and further heating to fractionate.
As pitch softening points increase, production becomes exponentially more difficult. The higher temperatures required to vaporize the high boiling diluent from the crude pitch require temperatures which are high enough that the pitch production apparatus can easily coke up unless heroic measures are taken to prevent coking. Hot surfaces, e.g., metal tubes in a fired heater, produce hot spots which induce thermal polymerization and coking. Injected air, an “in situ” combustion process, can generate heat without hot metal, but the air can degrade the product while burning some of it up
Pitch producers want high softening point pitches for myriad reasons. These materials have a high coking value, an essential pitch property for making carbon containing artifacts and carbon fibers. High softening point pitch materials, and intermediate products containing them, have greater mechanical strength, both during manufacture and in the finished product, as compared to like products made from lower softening point pitch. Higher carbon contents, in pitch and in products made from pitch, usually mean higher strength and better product performance. High softening point pitch is mostly carbon, and pitch value is like that of other forms of carbon, diamonds are denser, and more valuable, than graphite.
For over half a century pitch producers have sought higher softening point products. Some refiners operate pitch fractionators under vacuum (to reduce the temperatures required to vaporize volatiles). Some use a wiped film evaporator, which relies on thin films and brute force mechanical wiping to prevent the sticky pitch from staying too long in contact with a hot metal wall. Some inject inerts, such as steam, for agitation or to create a pseudo vacuum, or some combination of these approaches. Some inject air, letting in situ combustion make some of the heat. Various combinations of all of the above approaches have been tried, as refiners tried to get around an upper limit on pitch softening point which had been set by their equipment and/or approach to pitch fractionation.
At this point, processes which use special approaches to make high softening point pitch will be reviewed, to show just how much effort pitch refiners have expended toward making high softening point pitch products.
U.S. Pat. No. 2,768,119, filed Dec. 31, 1952, assigned to Phillips Petroleum, taught making petroleum pitch. An aromatic extract was prepared by solvent extraction, then the aromatic extract thermally cracked to produce a fuel oil fraction from which a pitch fraction was recovered by vacuum distillation. The patentee reported that pitch could be made from petroleum and had many of the properties of coal tar pitch. Vacuum distillation conditions included a “pressure of about 1 mm Hg, a temperature in the range 440 to 650° F. . . . ” The vacuum distillation removed sufficient volatile matter to produce a product with the desired softening point (188° F. to 240° F. was reported in the patent).
U.S. Pat. No. 3,928,170 taught injecting hot gas into heavy oil to make pitch.
U.S. Pat. No. 3,974 and U.S. Pat. No. 4,026,788, McHenry, taught pitch manufacture with inert gas sparging.
U.S. Pat. No. 3,976,729 and U.S. Pat. No. 4,017,327, Lewis, taught making pitch with agitation during heat treatment.
U.S. Pat. No. 4,039,423, assigned to Gulf Oil, taught heating, flashing and “oxy-activation” to make pitch.
U.S. Pat. No. 4,066,737, assigned to Koppers, describes an oxidative pitch process as part of a method of making carbon fibers.
U.S. Pat. No. 4,242,196 assigned, inter alia to Sumitomo Metal, taught heating a resid to 450-520° C. in a tubular heater for 0.5-15 minutes, then passing an inert gas at 400-2000° C. for direct contact heating for ½-10 hours, to make pitch.
U.S. Pat. No. 4,431,512, assigned to Exxon, taught heat soaking steam cracker tar middle distillate at 420-440° C. for 2-6 hours, then vacuum stripping. Their U.S. Pat. No. 4,427,530 disclosed a similar process using FCC bottoms as feed.
U.S. Pat. No. 4,673,486 taught treating a solvent de-asphalted fraction with a carrier gas, and thermally cracking at 400-600° C. to produce a gas oil fraction and a pitch product.
U.S. Pat. No. 4,999,099 taught use of an oxidative purge gas to make pitch. An FCC heavy resid fraction was heat soaked at 385° C., then subjected to an O2+N2 sparge.
U.S. Pat. No. 5,540,832, assigned to Conoco Inc., taught making mesophase pitch from refinery decant oil residue by heat soaking at 386° C. for 28 hours with N2 agitation.
Ashland Petroleum obtained a series of patents on high softening point pitches, primarily for manufacture of carbon fiber. U.S. Pat. No. 4,671,864 taught vacuum flashing, or use of a wiped film evaporator (WFE), to reduce residence time of pitch at high temperature and form a pitch having a softening point of about 250° C. U.S. Pat. No. 5,238,672 taught heating isotropic pitch with inert gas, at high temperature, to make mesophase pitch. U.S. Pat. No. 5,316,654 taught use of WFE to make high softening point pitch. U.S. Pat. No. 5,429,739 taught use of a thin film, reduced pressure and partial oxidation to make high softening point pitch, converting a conventional 250° F. softening point pitch to high softening pitch in a WFE. Partial oxidation sped up the process. U.S. Pat. No. 5,614,164 taught starting with a pitch with a softening point of 93-233° C., WFE processing for 115-300 seconds to produce “enriched pitch” then stripping with an inert gas for up to 18 hours to produce pitch product, with a softening point of 177-399° C.
The Eureka® Process, developed by Kureha Chemical Industry Co. Ltd and Chiyoda, has been used for over 20 years to make pitch products. The process injects steam into the pitch forming reactor to create a pseudo vacuum and keep the molten pitch as a homogeneous liquid.
Although not related to pitch production, mention will be made at this point of use of molten metal baths, to dry paper pulp, in U.S. Pat. No. 5,619,806, Drying of Fiber Webs, Warren. The patentee used an alloy composition of bismuth and zinc. Molten metal baths are also used for metal plating and for the float process to make plate glass.
All of the patents discussed or referred to above, and hereafter, are expressly incorporated by reference, in their entirety.
I reviewed these multiple routes to pitch products, especially to high softening point pitch products and found none completely satisfactory. I realized that much of the difficulty of processing pitch to fractionate it to increase its softening point was inherent in the material. The same sticky properties which made it a theoretical super glue for soldier's feet made it a bear to process using conventional technology. I did not want to burn or oxidize product to make higher softening point pitch (oxygen or air injection). I did not want to use conventional hot metal surfaces to heat viscous pitch products or precursors sufficiently to distill vaporizable components. Fired heaters, with their hot metal surfaces had a cursed “Midas touch” for such sticky tars and rapidly initiated thermal polymerization, further reducing viscosity. The viscous, sticky material would cling even longer to metal surfaces and the long residence time lead to coking, which further reduced the viscosity. This problem, of things getting even stickier because of contact with hot metal, has been around for millennia, and the usual remedy—constant stirring—helped, but only up to a point. As temperatures, and pitch softening points, increased, coking tendencies increased, so that resort was made to expensive and mechanically complicated stirring systems, wiped film evaporators. I wanted to avoid the high cost of continuous stirring, the high capital and operating cost, and limited throughput, of wiped film evaporator technology.
While I wanted a better route to high softening point pitch, I was also interested in improving existing pitch processes, whether based on wood, coal or petroleum pitch.
In reviewing the problems of pitch fractionation processes, which have been around for millennia, I discovered an entirely new way to fractionate crude pitch which completely avoided the problems associated with prior processes.
I realized that a technology used for decades to make plate glass (forming glass on a bed of molten metal), could overcome the heating barrier imposed by solid metal heating surfaces. If molten glass does not stick to molten metal, neither would molten pitch. I used a molten liquid heating medium which would heat pitch but to which pitch would not stick. Using molten metal, it was possible to heat pitch sufficiently to vaporize volatile components and avoid the sticking problem.
The molten metal bath was wonderfully efficient at heating the pitch. Molten metal was relatively free of hot or cold spots, because of its high thermal conductivity. Most important, crude pitch does not stick to molten metal, eliminating the sticking and coking problem associated with hot metal.
Molten metal also permits a flexible design approach, permitting injection of the metal into the oil or vice versa, though not necessarily with equivalent results. When pitch is injected into a molten metal bath, it is possible to increase or decrease to some extent fractionation severity by changing the depth of molten metal in the bath, the temperature of the metal, the pressure in the molten metal bath or the presence of a stripping gas to create a “pseudo vacuum”, or some combination of these. For the first time, pitch producers have many more degrees of freedom to pursue the best pitch product, in a process which is wonderfully tolerant of mistakes. While mistakes may be made, they will not stick to the molten metal, so a pitch fractionator can generally continue in operation even if some coke solids are produced by mistake.